Decorator-based architecture in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS controller method?

Consider this NestJS controller method decorated with @Get and @UseGuards. What will be the HTTP response status code when this endpoint is accessed successfully?

NestJS

import { Controller, Get, UseGuards } from '@nestjs/common';
import { AuthGuard } from '@nestjs/passport';

@Controller('items')
export class ItemsController {
  @Get()
  @UseGuards(AuthGuard('jwt'))
  findAll() {
    return { message: 'Items fetched' };
  }
}

A 200 OK with JSON { message: 'Items fetched' }

B 401 Unauthorized error

C 404 Not Found error

D 500 Internal Server Error

💡 Hint

Think about what @UseGuards(AuthGuard('jwt')) does when the user is authenticated.

The @Get decorator defines an HTTP GET endpoint. The @UseGuards with AuthGuard('jwt') protects the route, allowing only authenticated requests. If authentication succeeds, the method returns the JSON with status 200 OK.

📝 Syntax

intermediate

Which option correctly applies a custom decorator to inject a user object in NestJS?

You want to create a custom decorator @CurrentUser() that extracts the user from the request object. Which option correctly defines and uses this decorator in a controller method?

NestJS

import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export const CurrentUser = createParamDecorator((data: unknown, ctx: ExecutionContext) => {
  const request = ctx.switchToHttp().getRequest();
  return request.user;
});

@Controller('profile')
export class ProfileController {
  @Get()
  getProfile(@CurrentUser() user) {
    return user;
  }
}

A Correct as shown in the code above

B Replace createParamDecorator with @Injectable decorator

C Use @Param('user') instead of @CurrentUser()

D Define CurrentUser as a class instead of a function

💡 Hint

Custom parameter decorators use createParamDecorator function.

The createParamDecorator function is the correct way to create a custom parameter decorator in NestJS. It accesses the request object via ExecutionContext and returns the user property.

🔧 Debug

advanced

Why does this NestJS service fail to inject the repository?

Given this service code, why will NestJS throw a runtime error about missing dependencies?

NestJS

import { Injectable } from '@nestjs/common';
import { InjectRepository } from '@nestjs/typeorm';
import { Repository } from 'typeorm';
import { User } from './user.entity';

@Injectable()
export class UserService {
  constructor(@InjectRepository(User) private userRepository: Repository<User>) {}

  findAll() {
    return this.userRepository.find();
  }
}

A The findAll method is missing async keyword

B The @Injectable decorator is missing

C The User entity is not imported correctly

D The constructor parameter is missing @InjectRepository(User) decorator

💡 Hint

Check how TypeORM repositories are injected in NestJS services.

When injecting a repository, you must use @InjectRepository(Entity) on the constructor parameter. Without it, NestJS cannot resolve the dependency.

❓ lifecycle

advanced

What happens when a NestJS provider implements OnModuleInit?

If a service class implements the OnModuleInit interface and defines the onModuleInit() method, when is this method called?

NestJS

import { Injectable, OnModuleInit } from '@nestjs/common';

@Injectable()
export class AppService implements OnModuleInit {
  onModuleInit() {
    console.log('Module initialized');
  }
}

A Called every time a request is received

B Called once after the module's providers are initialized

C Called only when the application shuts down

D Called before the module's providers are created

💡 Hint

Think about the lifecycle of modules and providers in NestJS.

The onModuleInit() method is called once after all providers in the module have been initialized, allowing setup logic after injection.

🧠 Conceptual

expert

Which option best describes the role of decorators in NestJS architecture?

In NestJS, decorators like @Controller, @Injectable, and @Module are used extensively. What is their primary purpose in the framework's architecture?

A They execute code immediately to modify class behavior at runtime

B They replace the need for writing any configuration files

C They add metadata to classes and methods to enable dependency injection and routing

D They automatically generate database schemas from classes

💡 Hint

Think about how NestJS uses decorators to organize and connect parts of the app.

Decorators in NestJS add metadata that the framework reads to perform dependency injection, routing, and module organization. They do not execute code immediately or generate schemas automatically.

Practice

1. In NestJS, what is the main purpose of using decorators like @Controller() and @Injectable()?

easy

A. To replace the need for writing any class methods

B. To execute the class methods automatically on app start

C. To mark classes with specific roles and add metadata for NestJS to manage

D. To style the output of the application in the browser

Solution

1. Step 1: Understand decorator roles in NestJS

   Decorators like @Controller() and @Injectable() add metadata to classes, telling NestJS their purpose.

2. Step 2: Recognize how NestJS uses this metadata

   NestJS uses this metadata to organize and manage components like controllers and services during runtime.

3. Final Answer:

   To mark classes with specific roles and add metadata for NestJS to manage -> Option C

4. Quick Check:

   Decorators add metadata = C [OK]

Hint: Decorators label classes for NestJS to organize [OK]

Common Mistakes:

• Thinking decorators run methods automatically
• Confusing decorators with styling tools
• Believing decorators remove the need for methods

2. Which of the following is the correct way to define a controller class in NestJS using decorators?

easy

A. @Controller() class MyController { }

B. class MyController { }

C. function Controller() { } class MyController { }

D. @Injectable() class MyController { }

Solution

1. Step 1: Recall the syntax for controller decorators

   In NestJS, the @Controller() decorator is placed immediately before the class definition.

2. Step 2: Check each option for correct decorator usage

   @Controller() class MyController { } correctly shows @Controller() before the class. class MyController { } lacks any decorator. function Controller() { } class MyController { } incorrectly defines a function instead of using a decorator. @Injectable() class MyController { } uses @Injectable() which is for services, not controllers.

3. Final Answer:

   @Controller() class MyController { } -> Option A

4. Quick Check:

   Controller decorator before class = B [OK]

Hint: Controller classes need @Controller() decorator above class [OK]

Common Mistakes:

• Omitting the decorator entirely
• Using @Injectable() instead of @Controller()
• Trying to define decorators as functions

3. Given the following NestJS controller code, what will be the output when a GET request is made to '/hello'?

@Controller('hello')
export class HelloController {
  @Get()
  greet() {
    return 'Hello, NestJS!';
  }
}

medium

A. "Hello, NestJS!"

B. 404 Not Found error

C. An empty response

D. A runtime error due to missing decorator

Solution

1. Step 1: Analyze the route and method decorators

   The @Controller('hello') sets the base path to '/hello'. The @Get() decorator on greet() means this method handles GET requests to '/hello'.

2. Step 2: Understand the method return value

   The greet() method returns the string 'Hello, NestJS!'. So, a GET request to '/hello' returns this string as the response.

3. Final Answer:

   "Hello, NestJS!" -> Option A

4. Quick Check:

   GET /hello returns method string = A [OK]

Hint: Match @Controller path with @Get method for output [OK]

Common Mistakes:

• Assuming 404 because of missing route
• Expecting JSON instead of string
• Thinking decorators cause runtime errors here

4. Identify the error in the following NestJS service code:

@Injectable
export class UserService {
  getUser() {
    return { name: 'Alice' };
  }
}

medium

A. Return value must be a string, not an object

B. Missing parentheses in @Injectable decorator

C. Class name should be UserServices

D. Method getUser() should be async

Solution

1. Step 1: Check the decorator syntax

   The @Injectable decorator requires parentheses even if no arguments are passed, so it should be @Injectable().

2. Step 2: Verify other code parts

   The method getUser() can be synchronous and return an object. Class name and return type are valid as is.

3. Final Answer:

   Missing parentheses in @Injectable decorator -> Option B

4. Quick Check:

   @Injectable() needs parentheses = D [OK]

Hint: Always add () after @Injectable even if empty [OK]

Common Mistakes:

• Omitting parentheses on decorators
• Thinking methods must be async
• Changing class names unnecessarily

5. You want to create a NestJS controller that handles GET requests to '/users/:id' and returns the user ID from the URL. Which of the following code snippets correctly uses decorators to achieve this?

hard

A. @Controller('users')
export class UserController {
@Get(':id')
getUser(id: string) {
return `User ID is ${id}`;
}
}

B. @Controller('users')
export class UserController {
@Get()
getUser(@Body('id') id: string) {
return `User ID is ${id}`;
}
}

C. @Controller('users')
export class UserController {
@Post(':id')
getUser(@Param('id') id: string) {
return `User ID is ${id}`;
}
}

D. @Controller('users')
export class UserController {
@Get(':id')
getUser(@Param('id') id: string) {
return `User ID is ${id}`;
}
}

Solution

1. Step 1: Identify the correct HTTP method and route parameter usage

   The route is '/users/:id' and should respond to GET requests, so @Get(':id') is correct.

2. Step 2: Check parameter decorator usage

   The method parameter must use @Param('id') to extract the 'id' from the URL path.

   @Controller('users')
   export class UserController {
     @Get(':id')
     getUser(@Param('id') id: string) {
       return `User ID is ${id}`;
     }
   }

   does this correctly.

3. Step 3: Verify other options

   @Controller('users')
   export class UserController {
     @Get()
     getUser(@Body('id') id: string) {
       return `User ID is ${id}`;
     }
   }

   uses @Body('id') which reads from request body, not URL.

   @Controller('users')
   export class UserController {
     @Post(':id')
     getUser(@Param('id') id: string) {
       return `User ID is ${id}`;
     }
   }

   uses POST instead of GET.

   @Controller('users')
   export class UserController {
     @Get(':id')
     getUser(id: string) {
       return `User ID is ${id}`;
     }
   }

   misses the @Param decorator, so 'id' won't be injected.

4. Final Answer:

   The code snippet with @Get(':id') and @Param('id') -> Option D

5. Quick Check:

   GET + @Param('id') for URL param = A [OK]

Hint: Use @Get(':id') and @Param('id') to get URL parameters [OK]

Common Mistakes:

• Using @Body instead of @Param for URL params
• Using wrong HTTP method decorator
• Forgetting @Param decorator on method argument